Embodiments of the present invention generally relate to a system and method for controlling the temperature of exhaust gas. More particularly, but not exclusively, embodiments of the present invention relate to an engine system that is adapted to control the temperature of an exhaust gas by modifying an engine speed and outputted engine power relationship as a function of compressor inlet air density.
Selective catalytic reduction systems (SCR) typically are configured to provide one or more catalyst elements that, with the aid of a reductant, convert nitrogen oxides (NOx) in exhaust gases into nitrogen (N2) and water. The reductant may be injected into the exhaust gas upstream of an SCR catalyst. Typically, engine after-treatment systems attempt to inject a sufficient quantity of reductant into the exhaust gas stream necessary for the conversion of a predetermined amount of the NOx in the exhaust gas stream so as to prevent NOx slippage without incurring reductant slippage.
The efficiency of chemical reactions by an SCR catalyst may depend on a variety of different factors, including, for example, at least on the properties of the exhaust gas. For example, the efficiency of a catalyst of a selective catalytic reduction system (SCR) in reducing nitrogen oxides (NOx) in an exhaust gas stream may depend, at least in part, on an inlet temperature and/or velocity of the exhaust gas steam that enters into the SCR. For example, FIG. 1 illustrates the conversion efficiency of an SCR in reducing NOx in an exhaust gas stream as a function of the inlet temperature of the exhaust gas stream entering into, received by, or at the catalyst of the SCR (SCR inlet exhaust gas temperature). As shown, in this example, the maximum NOx conversion efficiency, as a function of reaction kinetics such as temperature and space velocity, is attained when the SCR inlet temperature is from around 300 degrees Celsius to around 450 degrees Celsius. However, engine operations that experience low compressor inlet air density conditions, such as, for example, during high altitude and/or high temperature operating conditions, the SCR inlet exhaust gas temperature often exceeds the temperatures at which maximum NOx conversion efficiency may be attained by the SCR. As a consequence, under such operating conditions, the emissions released from the after-treatment system may exceed a target, or regulation specified, amount. Further, the relatively high SCR inlet exhaust gas temperatures that may be attained when engine operations experience low compressor inlet air density conditions may damage at least the SCR system.